There’s just one thing concerning the reliability of heat packs: I am not sure whether the failure in warming the parcel adequately might be due to wrong usage.
I experienced heat packs to be cold when unpacking a parcel, but after kneading and exposing them to air they began to give off heat.
It’s necessary to activate a heat pack by kneading it – and it’s unalterable to care for sufficient air supply while packed inside the parcel. It’s a physical reaction that produces warmth, it works through oxidation. If the heat pack is wrapped or enclosed too tightly it doesn’t work. It’s a balancing act between keeping the greatest heat away from direct contact to the plastic bags and still not wrapping the heat pack so tight that it is cut off from oxygen supply.
Looking for a method to safely use heat packs I found the advice to take an egg carton (those molded ones made from paper pulp, take the size made for ten eggs), tear off one mold, put the activated heat pack in, close it and place it upright in between the plastic bags. It is said to cause a stack effect, the heat pack gets enough air to work, and the egg carton guarantees a safe distance from the plastic bags.
I haven’t tried it, but it sounds plausible to me.